This invention pertains to a novel emulsion polymerization process for the production of fluoroelastomers wherein a certain class of partially fluorinated anionic surfactants is used as the dispersing agent.
Fluoroelastomers having excellent heat resistance, oil resistance, and chemical resistance have been used widely for sealing materials, containers and hoses.
Production of such fluoroelastomers by emulsion and solution polymerization methods is well known in the art; see for example U.S. Pat. Nos. 4,214,060 and 4,281,092. Generally, fluoroelastomers are produced in an emulsion polymerization process wherein a water-soluble polymerization initiator and a relatively large amount of surfactant are employed. The surfactant most often used for such processes has been ammonium perfluorooctanoate (C-8). The resulting fluoroelastomer leaves the reactor in the form of a dispersion.
While C-8 works very well as a surfactant in the polymerization process, it is relatively expensive, and its future commercial availability is uncertain. Thus, it would be desirable to find other surfactants for use in the emulsion polymerization of fluoroelastomers.
Khan (U.S. Pat. No. 4,524,197) discloses an emulsion polymerization process for manufacturing fluoroelastomers. The process employs a surfactant of the formula Fxe2x80x94(CF2CF2)nCH2CH2xe2x80x94OSO3M, where n is an integer from 2-8 or mixtures thereof, and M is an alkali metal cation, hydrogen ion or ammonium ion. This surfactant has several disadvantages vs. C-8. First of all, the Khan surfactant acts as a chain transfer agent due to the active hydrogen atoms adjacent to the xe2x80x94OSO3-group, thus limiting the molecular weight of the fluoroelastomer. In addition, it is difficult to remove residual amounts of this surfactant from the resulting fluoroelastomer. Residual surfactant may interfere with vulcanization of the elastomer with such commonly employed curatives as bisphenols.
Khan and Morgan (U.S. Pat. No. 4,380,618) disclose an emulsion polymerization process for manufacturing crystalline thermoplastic tetrafluoroethylene homopolymers and copolymers. The process employs a surfactant of the formula Fxe2x80x94(CF2CF2)nCH2CH2xe2x80x94SO3M, where n is an integer from 2-8, or mixtures thereof, and M is an alkali metal cation, hydrogen ion or ammonium ion. The crystalline fluoropolymers produced in Khan and Morgan do not absorb surfactant to any substantial degree. In contrast, because fluoroelastomers are amorphous, rather than crystalline, it would be expected that this surfactant would be absorbed into fluoroelastomer polymer particles and thus be i) less effective in the polymerization of fluoroelastomers due to chain transfer limiting elastomer molecular weight and ii) difficult to wash out of any fluoroelastomer produced, thus adversely effecting vulcanization.
Blaise and Grimaud (U.S. Pat. No. 4,025,709) disclose a similar process for the production of crystalline thermoplastic vinylidene fluoride homopolymers and copolymers. The process employs a surfactant of the formula Rfxe2x80x94CH2CH2xe2x80x94SO3M, where Rf is a perfluorinated radical having 4 to 10 carbon atoms and M is an alkali metal cation, or ammonium ion. As in Khan and Morgan, the crystalline fluoropolymers produced in Blaise and Grimaud do not absorb surfactant to the same degree that an amorphous fluoroelastomer would when made in a process employing the above surfactant. Thus this surfactant would be expected to exhibit the same deficiencies as the surfactants disclosed in U.S. Pat. No. 4,380,618 when employed in a polymerization process for making fluoroelastomers.
Baker and Zipfel (U.S. Pat. Nos. 5,789,508 and 5,688,884) disclose that the polymerization process of Khan and Morgan for crystalline thermoplastic tetrafluoroethylene homopolymers and copolymers is improved when the surfactant employed is a particular species of the genus used by Khan and Morgan. The preferred surfactant is of the formula C6F13CH2CH2SO3M wherein M is a cation having a valence of 1. The latter is less toxic than other species of the genus. For the same reasons as mentioned in the discussion of Khan and Morgan patent above, one skilled in the art would not predict that the Baker and Zipfel surfactant would be satisfactory in a process for making fluoroelastomers.
One aspect of the present invention provides an emulsion polymerization process for the production of fluoroelastomers comprising:
(A) charging a reactor with a quantity of an aqueous solution comprising a surfactant of the formula Fxe2x80x94(CF2CF2)nxe2x80x94CH2CH2SO3M where n is an integer from 2 to 9, or mixtures thereof, and M is a cation having a valence of 1;
(B) charging the reactor with a quantity of a monomer mixture to form a reaction medium, said monomer mixture comprising i) from 25 to 70 weight percent, based on total weight of the monomer mixture, of a first monomer, said first monomer selected from the group consisting of vinylidene fluoride and tetrafluoroethylene, and ii) between 75 and 30 weight percent, based on total weight of the monomer mixture, of one or more additional copolymerizable monomers, different from said first monomer, wherein said additional monomer is selected from the group consisting of fluorine-containing olefins, fluorine-containing vinyl ethers, hydrocarbon olefins and mixtures thereof; and
(C) polymerizing said monomers in the presence of a free radical initiator to form a fluoroelastomer dispersion while maintaining said reaction medium at a pH between 1 and 7, at a pressure between 0.5 and 10 MPa, and at a temperature between 25xc2x0 C. and 130xc2x0 C.
Another aspect of the present invention is a curable fluoroelastomer prepared by the process of this invention.